Usually t is used for time. Plug in the correct value for t. Which is 5 hours.
w = -500(5) + 7000 ; Start
w = -2500 + 7000 ; Multiple -500 by 5
w = 4500 ; Subtract 2500 from 7000
So, after 5 hours of draining the pool, 4500 liters (I choose liters. It didn't specify in the question) of water are still in the pool.